The localization of macroscopic atherosclerotic lesions have been studied in the basilar bifurcation and the left and right internal carotid bifurcations in humans. The distribution of lesions was separately assessed by a computer technique and results were displayed as contour lines. Lesions are most frequent on the outer walls of the bifurcations and on the inner curvature of the arterial bends. The material was processed for histology, and computerised morphometry was used to study the interrelationship of microscopic atheromatous lesions and intimal thickening. The percentage of the specimens with lesions around the circumference of the arteries was calculated. The peaks and troughs The peaks and troughs of histological lesions correspond exactly to the maximum and minimum contours of the macroscopic studies. Results from the microscopic examination indicate that intimal thickening and atherosclerosis are closely related processes. However, they cannot support the view that focal pad-like areas of intimal thickening are particular precursors of atheroma.